Static power converters comprising a number of thyristor bridges driven to full output and capable of operating as rectifiers or, if necessary, as inverters are used in A.C. powered vehicles. In vehicles equipped with converters of this type, propulsion as well as regenerative braking is possible, and this constitutes an advantage from the point of view of saving energy.
In the regenerative-braking mode, the drive motors of the vehicle during the deceleration of the latter work as generators, and in the above-described application the D.C. power generated by the motors is converted into A.C. power. The converted A.C. power is fed to the overhead conductor. While this takes place, the phase shift control angle .alpha. of each thyristor must be limited to a maximum phase angle of 180.degree. - .gamma., so that the thyristor operates with a safety angle (.gamma. denotes the minimum control advance angle). Neglecting the overlap angle, .gamma. equals the safety angle. Hereinafter, the safety angle will be referred to as .gamma.. The overlap angle will be disregarded for the sake of simplification. The angle .gamma. is usually between 40 and 50.degree., to enable the thyristor to cease conducting.
The limitation of the phase shift control angle .alpha. introduces the disadvantage that the harmonics in the A.C. circuit become greater than in the case of a powered vehicle operating with a number of rectifier bridges driven to only half their output and used to control propulsion but not regenerative braking. These harmonics induce noise voltages which should be avoided as far as possible in telecommunication cables laid parallel with the overhead conductor.